1. Field of the Invention
The present invention relates to transducers, and in particular to transducers comprising electrical circuits for generating electrical signals in response to the application of force or energy to force or energy sensitive devices in the circuit.
2. Description of the Prior Art
Transducers for generating electrical signals in response to the application of force or energy to the transducer are well known basic electrical circuit components. Some well known means for providing transducer functions utilize changes in resistance, capacitance, inductance (including magnetism) or optical characteristics as a function of applied forces or changes in applied energy. Many of the known transducers are very accurate and effective in use. However, prior transducers have several serious shortcomings. One such shortcoming is that the ratio of their input power to the output signal is very high. The practical consequence of this is that present transducers require relatively high power inputs in order to obtain output signals which can be used in the circuitry associated with the transducer. Another serious shortcoming of known transducers, even those used in miniaturized electrical systems for detecting small changes in applied forces, is the high cost of such transducers. In fact, it is not uncommon for a transducer in a miniaturized electrical system to be much more expensive than the other components of the system.
It is a known phenomenon that the resistance of elements comprising particulate conductive material such as carbon granules, disposed in close proximity such as by being suspened in a resilient, non-conductive binder material like rubber, polyethelene, or the like, will vary in response to the application of force to the element. U.S. Pat. Nos. 2,690,489 (Jarret et al.), 3,341,797 (Watson), 3,398,233 (Lizasoain et al), 3,451,032 (Temple), 3,509,296 (Harshman et al.) and 3,820,529 (Gause et al,) disclose devices employing this phenomenon. Such devices can be used as transducers because their resistances are a function of applied pressure, and thus effect changes in voltage or current when they are connected to appropriate power sources and are subjected to such pressure. Nevertheless, this phenomenon has not been successfully exploited to achieve inexpensive transducers having low ratios of power input to signal output.
The balanced bridge circuit is well known as a basic transducer circuit. Balanced bridge circuits usually comprise four interconnected legs, each having a resistor element, with a power source being connected across the junctures of opposite pairs of the legs, and the output being connected across the other two junctures of the legs. At least one of the legs is sensitive to applied force (or energy) with the resistance in that leg varying in response to the application of force (or energy). (Although the transducers discussed herein are described as being force responsive, it is intended that such responsiveness applies to all energy applied to the transducer, such as for example thermal energy). When the balanced bridge is in its unstressed, stable condition, there is no output signal. When force to which the sensitive element is responsive is applied, either of a positive or negative sense, a corresponding output signal is generated by the bridge circuit.
In order to achieve the desired responsiveness of the bridge circuit, it is important that the circuit have a predetermined fixed output (usually zero) in its unstressed, stable condition, and the latter is theoretically achieved by balancing the resistances of the component elements. The balanced bridge circuit is in theory particularly susceptible to such balancing, particularly to achieve compensation for changes in temperature and for achieving linearity of the output signal in response to the input force.
Even though such bridge circuits are in theory balanced after being adjusted, these circuits nonetheless have an inherent tendency towards a state of imbalance. This is largely due to variations which are known to occur in the impedance of individual resistive elements in response to changes of temperature, force, or aging. Therefore, balanced bridge circuits generally require extraordinary measures to correct for these factors if their response to applied forces or energy is to remain acceptably accurate under all prevailing conditions.
Another inherent shortcoming of prior balanced bridge circuits is that they are characterized by an initial non-linear response to applied forces, this "non-linear area" often being of little consequence when the applied force or energy is of the type which would not fall within that area. However, for bridge circuits which are intended to measure very small forces, and especially in the case of the measurement of small changes in external forces, this non-linearity can severely limit the accuracy of the bridge circuit. The elimination of such "non-linear area" would be very advantageous for many applications of bridge circuits.